1. Field of the Invention
The present invention relates to an improved process for the liquefaction of raw coal. More particularly, the invention relates to a three-stage process wherein solvents, having differing quantities of donatable hydrogen, are used to minimize gas yields and hydrogen consumption.
2. Prior Art
Coal is our most abundant indigenous fossil fuel resource, and as a result of dwindling petroleum reserves, concerted research efforts are being directed toward recovery of liquid hydrocarbons from coal on a commercial scale. A promising approach in this field is the direct liquefaction of coal.
This approach has principally evolved from the early work of F. Bergius, who discovered that transportation fuels could be produced by the high pressure hydrogenation of a paste of coal, solvent and catalyst.
Later discoveries revealed the advantage of using specific hydrogenation solvents at lower temperatures and pressures. With these solvents, such as partially saturated polycyclic aromatics, hydrogen transfer to the coal is facilitated and dissolution enhanced. However, the products from single-stage dissolvers are typically high in asphaltenes, have high average molecular weights and high viscosities. These qualities present considerable obstacles in removing the final coal residue particles suspended in the product which usually range from 1 to 25 microns in diameter.
The complete nature of the coal residue or undissolved solids is not fully understood, but the residue appears to be a composite of organic and inorganic species. The residue organic matter is similar to coke and the inorganic matter is similar to the well known coal-ash constituents. The removal of these particles is, of course, necessary to produce a clean-burning, low-ash fuel.
Direct two-stage coal liquefaction processing evolved by the addition of a catalytic stage to further hydrogenate and break down the higher molecular weight products produced in the dissolver. In retrospect, and with the clarity hindsight often provides, such a step does not seem unprecedented. However, the direct passage of a solids-laden stream through a catalytic reactor was theretofor considered impractical at best. The two-stage units solved most of the coal residue removal problems since the hydrocracked productwas relatively light and of relatively low viscosity, thereby permitting the use of conventional solids removal techniques and the asphaltene content of the product from the catalytic reactor was drastically reduced by the catalytically induced hydrogenation. Representative patents covering staged coal liquefaction processes include U.S. Pat. No. 4,018,663 issued to C. Karr, Jr. et al, U.S. Pat. No. 4,083,769 issued to R. Hildebrand et al and U.S. Pat. No. 4,111,788 issued to M. Chervenak et al.
U.S. Pat. No. 4,018,663 discloses a two-stage process in which a coal-oil slurry is passed through a first reactor containing a charge of porous, non-catalytic contact material in the presence of hydrogen at a pressure of 1,000 to 2,000 psig and a temperature of 400.degree. to 450.degree. C. The effluent from this reactor is then preferably filtered to remove the coal residue and passed to a catalytic reactor for defulfurization, denitrification and hydrogenation of the dissolved coal.
U.S. Pat. No. 4,083,769 discloses a process wherein a preheated coal-solvent slurry is passed with hydrogen through a first dissolver zone operated at a pressure in excess of 210 atmospheres and at a higher temperature than the preheater. The dissolver effluent is then hydrogenated in a catalytic zone also maintained at a pressure in excess of 210 atmospheres and at a temperature in the range of 370.degree. to 440.degree. C. to produce liquid hydrocarbons and a recycle solvent.
U.S. Pat. No. 4,111,788 discloses a process wherein a coal-oil slurry is passed through a dissolver containing no catalyst and the effluent therefrom is subsequently treated in a catalytic ebullated bed at a temperature at least 14.degree. C. lower than the temperature of the dissolver. A portion of the product liquid is preferably recycled for use as solvent.
In each of the above processes, the coal is dissolved at high temperatures in the presence of hydrogen and/or a hydrogen-donor solvent. While the physical coal dissolution requires such temperatures, the residence times required for hydrogen transfer, coupled with the high temperatures, increase the overall gas yields at the expense of liquid product and increase hydrogen consumption.
It is therefore an object of this invention to provide a coal liquefaction process which maximizes the liquid product yields without sacrificing product quality.